Coupled-cavity Q-switched laser

ABSTRACT

A Q-switched laser having a gain medium disposed within a first cavity and a second cavity whose optical path length is adjustable such that the quality of the first resonant cavity is affected. One aspect of the invention is the changing of the physical path length of the second cavity so as to effect the reflectivity of a mirror common to both cavities as seen from the first cavity. Another aspect of the invention is the incorporation, within the second cavity, of a material whose refractive index or absorption coefficient can be varied by the application of an electric field, a magnetic field, a temperature change or an applied pressure.

The Government has rights in this invention pursuant to Contract NumberF19628-85-C-0002 awarded by the Office of the Air Force.

BACKGROUND OF THE INVENTION

This invention relates to the field of lasers. Many applications requirethe generation of short pulses of light from a laser at a highrepetition rate. One method of producing a rapid pulse of light is toQ-switch the laser. In Q-switching, the "quality" of the laser cavity ischanged. One method for changing the quality of the cavity is tomechanically move one of the cavity mirrors into and out of alignmentwith the other mirror of the cavity. When the mirror is out ofalignment, there is no resonant cavity and no lasing can occur. When themirror is moved into alignment, the resonant cavity is formed and lasingbegins. The large motions required to move the mirror into and out ofalignment limit the rate at which the laser can be Q-switched.

Other Q-switching techniques allow rapid Q-switching of the laser, butrequire large intracavity devices which are incompatible with shortcavity length lasers.

The present invention changes the Q of the resonant cavity in ways thatpermit rapid switching of the laser, and is particularly well suited foruse with lasers having short cavity lengths.

SUMMARY OF THE INVENTION

The Q-switched laser according to the invention comprises a gain mediumdisposed within a first resonant cavity and a second resonant cavitydisposed adjacent to the first resonant cavity and sharing a commonpartially transmitting mirror with said first resonant cavity. Theoptical path length of the second resonant cavity (physical length timesrefractive index) is adjustable such that the Q of the first resonantcavity containing the gain medium is affected.

In one embodiment, the optical path length of the second resonant cavityis varied by moving an output mirror.

In another embodiment, the second resonant cavity comprises anelectro-optical material whose index of refraction varies in response toan externally applied electric field.

In yet another embodiment, the material within the second resonantcavity is a non-linear optical material which changes its index ofrefraction is response to a second light beam incident upon thenon-linear optical material.

In still yet another embodiment the optical material disposed within thesecond cavity changes its index of refraction is response to atemperature change.

In further yet another embodiment, the optical material located withinthe second cavity changes its index of refraction in response to amagnetic field.

In yet another embodiment, the second resonant cavity comprises anelectro-optical material whose absorption coefficient changes inresponse to an externally applied electric field.

In a still further embodiment, the second resonant cavity furthercomprises an external coupling mirror whose reflectivity changes inresponse to a potential placed across the mirror.

In another embodiment, the net gain of the first cavity is reduced bychanging the optical path length of the second cavity such that theintensity of the laser light is diminished but not turned off.

BRIEF DESCRIPTION OF THE DRAWING

A more complete understanding of the invention may be obtained from thefollowing detailed description when taken in conjunction with thedrawings, in which:

FIG. 1(a) is a cross-sectional view of an embodiment of a laser known tothe prior art, FIG. 1(b) is a graph which depicts the intensity versesfrequency diagram for the laser of FIG. 1(a), FIG. 1(c) is across-sectional view which depicts an embodiment of the inventionwherein the laser of FIG. 1(a) is adjacent to a second cavity having amovable output coupling mirror, FIG. 1(d) is a graph of reflectivityverses frequency for a partially transparent mirror located between thegain medium and the second cavity of FIG. 1(c) as seen looking towardthe mirror from the gain medium;

FIG. 2(a) is a schematic diagram of an embodiment of the inventionwherein the second resonant cavity comprises an electro-opticalmaterial, FIG. 2(b) is a schematic diagram of an embodiment of theinvention wherein the second resonant cavity comprises a non-linearoptical material, FIG. 2(c) is a schematic diagram of an embodiment ofthe device wherein the second resonant cavity comprises amagneto-optical material;

FIG. 3 is a schematic diagram of an embodiment of the invention whereinthe reflectivity of the output coupling mirror is varied;

FIG. 4 is a schematic diagram of an embodiment of the invention whereinthe length of the gain cavity is varied; and

FIG. 5 is a schematic diagram of an embodiment of the invention in whichthe resonant cavity is placed between the gain medium and the pump lightsource.

DESCRIPTION OF THE PREFERRED EMBODIMENT Theory

Referring to FIG. 1(a), an active gain medium 10 disposed within acavity 8 formed by two mirror 12, 14, will lase 18 when excited by pumplight 16, provided the gain resulting from the passage of the lightbetween the mirrors 12, 14 exceeds the losses due to absorption,scattering and transmission through the mirrors. To prevent a laser fromlasing, it is only necessary to change the reflectivity of one of themirrors so that not as much light is reflected and thereby cause thegain in the medium to be exceeded by the loses.

An active gain medium 10 disposed within a cavity 8 formed of twomirrors 12 and 14 will produce laser light of a frequency determined bythe modes 20 (FIG. 1(b) of the cavity. That is, the frequency of thelaser light (ν) will correspond to one of the cavity modes, given by theequation:

    ν=mc/2nl

where m is an integer, c is the speed of light, n is the refractiveindex within the cavity, and l is the cavity length.

Referring to FIG. 1(c), if mirror 14 of resonant cavity 8 (FIG. 1(a))containing gain medium 10 is replaced by a second optical resonantcavity 30 formed by two partially reflecting mirrors 22 and 24, withmirror 22 common to gain cavity 8 and second resonant cavity 30, thereflectivity of the mirror 22 as seen by the gain medium 10 lookingtoward the second resonant cavity 30 is determined by the resonant modesof the second resonant cavity 30. Referring to FIG. 1(d), at frequenciescorresponding to the resonant frequencies of the second resonant cavity30, the reflectivity 28 of the mirror 22 seen by the gain cavity 8 willbe low, while away from the resonant frequencies of the second cavity30, the reflectivity will be high. The resonant modes of the secondresonant cavity 30 are determined by the optical length of the secondresonant cavity 30 and can be changed, for example, by changing thephysical length of the second cavity 30 by moving the output couplingmirror 24 is indicated by the double headed arrow 32.

It is therefore possible to prevent or permit the gain medium 10 to laseby adjusting the second resonant cavity 30 such that the resonances ofthe second cavity cause either low reflectivity in the common mirror,(therefore preventing lasing) or high reflectivity in the common mirror(therefore inducing lasing).

If the length of the gain cavity 8 is such that several longitudinalcavity modes can lase, the optical length of the second cavity 30 mustbe an integer multiple of the optical length of the gain cavity 8 if allthe longitudinal lasing modes are to be suppressed simultaneously. Ifthe gain cavity 8 supports only one possible lasing mode, then there isno such constraint on the second cavity.

Implementation

Referring again to FIG. 1(c), the optical length of the second resonantcavity 30 can be changed either by changing the refractive index of thecavity of by changing the cavity's physical length. The embodiment shownin FIG. 1(c) changes the physical length of the second resonant cavity30 by moving the output coupling mirror 24 in the direction of thecavity axis (indicated by the doubled headed arrow 32). Such a mirror 24can be moved piezoelectrically very rapidly thereby producing a seriesof laser pulses 18 as the resonance modes of the second cavity 30 causethe reflectivity of the mirror 22, as seen by the gain medium 10 in thegain cavity 8, to vary.

FIG. 2(a) shows another embodiment of the invention similar to that ofFIG. 1(c) except that the optical path length of the second cavity 30 ischanged not by moving the output coupling mirror 24 but instead byapplying a voltage between two electrodes 212 disposed adjacent to anelectro-optical material 210. An electro-optical material changes itsindex of refraction in response to an electric field, causing theminimum in the reflectivity versus frequency curve to shift, therebyvarying the reflectivity of the mirror 22 as seen by the gain medium 10in the gain cavity 8.

Examples of such electro-optical materials are the semiconductors, whichcan be engineered so as to produce a change in their index of refractionin a specific portion of the spectrum. For example,gallium-aluminum-arsenide is suitable for applications involving thenear infra-red region of the spectrum, while cadmium sulfide is suitablefor visible light applications. Further, lithium niobate and potassiumniobate are also used as electro-optical materials although lithiumniobate undergoes photodamage and as such may be unsuitable for extendeduse. Other materials with electro-optical properties are well known andare discussed in the Handbook of laser Science and Technology-Vol. 4,part 2, section 2, Special Properties Subsection 2.1, LinearElectro-Optical Materials, RC Press, Boca Raton, Fla. (1986)incorporated herein by reference.

In the embodiment shown schematically in FIG. 2(a), two electrodes 212are placed on opposite sides of an electro-optical material 210 disposedbetween the partially transmitting common mirror 22 and the outputcoupling mirror 24. The index of refraction of this electro-opticalmaterial 210 may be changed at a high frequency by applying a highfrequency electric field between the electrodes 212. The application ofa varying electric field results in the production of a series of outputpulses 18 as the reflectivity of the common mirror 22 seen by the gainmedium 10 changes due to the resonances of the second cavity.

Another embodiment of the invention is shown in FIG. 2(b). In thisembodiment, a non-linear optical material 220, such as one of thesemiconductors described in the previous embodiment, is disposed betweenthe mirrors 22 and 24, and changes it index of refraction in response toa second incident laser beam 222 directed othogonally at the material220. Materials with this non-linear optical property are discussed inthe Handbook of Laser Science and Technology-Vol. 3 Optical Properties,CRC Press, Boca Raton, Fla. (1986) incorporated herein by reference.Again the change in the index of refraction caused by the second beam222 results in a change in the reflectivity of the common mirror 22 asseen by the gain medium 10. In such a configuration one series of laserpulses from the beam 222 can be used to form another series of pulses18.

FIG. 2(c) shows an embodiment wherein the second resonant cavitycomprises a magneto-optical material 230, such as yttrium-iron-garnet,whose index of refraction changes in response to an externally appliedmagnetic field. An electro-magnet 232 adjacent to the material 230 canbe used to change the index of refraction of the medium and hence thereflectivity of the mirror 22 as seen from the gain cavity 8, asdescribed previously. Materials with this property, such as the magneticspinels and garnets, are discussed in the Handbook of Laser Science andTechnology, Vol. 4 Optical materials, part 2, section 2, SpecialProperties, subsection 2.2, Magneto-optic Materials, CRC Press, BocaRaton, Fla. (1986) incorporated herein by reference.

Since most materials change their length in response to temperaturechanges, temperature can be use to modify the optical path length. Othermaterials, such as semiconductors, which have an index of refractionwhich changes in response to temperature independent of physical lengthchanges, can also be used to change the reflectivity of the mirror 22 asseen by the gain medium 10 and hence the net gain of laser.

Since most materials also change their length in response to appliedpressure, pressure can also be used to change the optical length of anoptical material in the second resonant cavity, and hence thereflectivity of the mirror 22 as seen by the gain medium 10.Additionally, certain materials such as yttrium-aluminum-garnet have apressure dependent index of refraction and will change their index ofrefraction independent of the change in length of the material inresponse to pressure.

Referring to FIG. 3, the resonance of the second cavity 30 can also bechanged by adjusting the reflectivity of an output coupling mirror 310.In this schematic drawing of the embodiment electrodes 312 adjacent tothe mirror 310 apply an electric field across the mirror, therebychanging its reflectivity. Mirrors with this property are easilyfabricated using an etalon with an electro-optical material.

Referring again to FIG. 2(a), a further embodiment uses the change inthe absorption coefficient of an electro-optical or non-linear opticalmaterial 210 in the second resonant cavity 30 to broaden the resonancesof the second resonant cavity and thereby change the reflectivity of themirror 22 seen by the gain medium 10. In such an embodiment, anelectro-optical or non-linear optical material 210 such as asemiconductor operating near its band edge, is disposed within thesecond cavity 30. Such a material changes it absorption coefficient whenan external electric field is applied, thereby changing the Q of thesecond cavity, and hence the reflectivity of mirror 22 as seen by thegain medium 10.

Referring to FIG. 4, it should also be understood that the opticallength of the laser gain cavity 8 itself could be modified instead ofmodifying the optical length of the second cavity 30. In one suchembodiment, the resonances of the gain cavity 8 are changed byphysically changing the length of the cavity (as indicated by the doublearrow 410) by moving the mirror 12. It is the change in the relativepositions of the resonances of the gain cavity 8 and the second resonantcavity 30 that affect the reflectivity of the mirror 22 seen by the gainmedium 10.

In yet another embodiment, (FIG. 5) the second resonant optical cavity30 is disposed between a pump light source 510 and the gain medium 10and modulates the amount of pump light 16 striking the gain medium 10within the gain cavity 8. For example, a second resonant cavity 30containing an electro-optical material 514 whose index of refraction isvaried by a voltage applied between electrodes 512 can be used tomodulate the amount of pump light 16 striking the gain medium 10.

Finally, it should be realized that the second resonant optical cavityneed not affect the gain cavity so much that the lasing is turnedcompletely on or off. Instead, the second resonant cavity can be used tomodulate the intensity of the light produced by the gain medium and notsimply turn the laser on and off.

Having described a number of embodiments, those skilled in the art willrealize many variations are possible which will still be within thescope and spirit of the claimed invention. Therefore, it is theintention to limit the invention only as indicated by the scope of theclaims.

What is claimed is:
 1. A Q-switched laser comprising a gain mediumdisposed within a first resonant cavity and a second resonant cavitydisposed adjacent to the first resonant cavity, the optical path lengthof the second resonant cavity being adjustable such that the.Iadd.optical length of the second resonant cavity is an integermultiple of the optical length of the first resonant cavity and the net.Iaddend.gain of the first resonant cavity containing said gain mediumis affected through optical interactions with the second cavity.
 2. TheQ-switched laser of claim 1 wherein the optical path length of thesecond resonant cavity is varied by changes in the physical length ofthe second resonant cavity.
 3. The Q-switched laser of claim 1 whereinthe optical path length of the second resonant cavity is varied bychanging the index of refraction of a material within the secondresonant cavity.
 4. The Q-switched laser of claim 3 wherein saidmaterial within said second resonant cavity has an index of refractionwhich varies in response to an external stimulus applied to saidmaterial.
 5. The Q-switched laser of claim 4 wherein the material withinsaid second cavity comprises an electro-optical material whose index ofrefraction changes in response to an applied electric field.
 6. TheQ-switched laser of claim 5 wherein said electro-optical material is asemiconductor.
 7. The Q-switched laser of claim 6 wherein saidsemi-conductor is gallium-aluminum-arsenide.
 8. The Q-switched laser ofclaim 6 wherein said semi-conductor is cadmium sulfide.
 9. TheQ-switched laser of claim 5 wherein said electro-optical material islithium niobate.
 10. The Q-switched laser of claim 5 wherein saidelectro-optical material is potassium niobate.
 11. The Q-switched laserof claim 4 wherein the material within said second resonant cavitycomprises a non-linear optical material which changes its index ofrefraction in response to a light beam incident upon the non-linearoptical material.
 12. The Q-switched laser of claim 11 wherein saidnon-linear optical material is lithium niobate.
 13. The Q-switched laserof claim 11 wherein said non-linear optical material is potassiumniobate.
 14. The Q-switched laser of claim 11 wherein said non-linearoptical material is a semiconductor.
 15. The Q-switched laser of claim14 wherein said semiconductor is gallium-aluminum-arsenide.
 16. TheQ-switched laser of claim 14 wherein said semiconductor is cadmiumsulfide.
 17. The Q-switched laser of claim 4 wherein the materialdisposed within said second resonant cavity changes its index ofrefraction in response to a change in the temperature of the material.18. The Q-switched laser of claim 4 wherein said material located withinsaid second cavity changes its index of refraction in response to amagnetic field.
 19. The Q-switched laser of claim 18 wherein saidmaterial located within said second cavity which changes its index ofrefraction in response to a magnetic field is a magnetic spinel.
 20. TheQ-switched laser of claim 18 wherein said material located within saidsecond cavity which changes its index of refraction in response to amagnetic field is a magnetic garnet.
 21. The Q-switched laser of claim 1wherein said second resonant cavity comprises an electro-opticalmaterial whose absorption coefficient changes in response to an appliedelectric field.
 22. The Q-switched laser of claim 21 wherein saidelectro-optical material comprises a semiconductor operated near itsband edge.
 23. The Q-switched laser of claim 1 wherein said secondresonant cavity comprises an electro-optical material whose absorptioncoefficient changes in response to light.
 24. The Q-switched laser ofclaim 23 wherein said electro-optical material comprises a semiconductoroperated near its band edge.
 25. The Q-switched laser of claim 1 whereinsaid second resonant cavity further comprises an external couplingmirror whose reflectivity changes in response to an external stimulus tothe mirror.
 26. The Q-switched laser of claim 25 wherein said externalcoupling mirror whose reflectivity changes in response to an externalstimulus to the mirror comprises an etalon comprising an electro-opticalmaterial.
 27. The Q-switched laser of claim 1 wherein the intensity ofthe light produced by the first cavity is changed by changing theoptical path length of the second cavity such that the intensity of thelight produced by the first cavity is diminished but not turned off. 28.A Q-switched laser comprising a gain medium disposed within a firstresonant cavity and a second resonant cavity disposed adjacent to thefirst resonant cavity, the optical path length of the first resonantcavity being adjustable such that the .Iadd.optical length of the secondresonant cavity is an integer multiple of the optical length of thefirst resonant cavity and the .Iaddend.net gain of the first resonantcavity containing said gain medium is affected through opticalinteractions with the second cavity.
 29. A Q-switched laser comprising again medium disposed within a first resonant cavity and a secondresonant cavity disposed adjacent to the first resonant cavity, betweenthe first resonant cavity and an optical pump, the optical bath lengthof the second resonant cavity being adjustable such that the.Iadd.optical length of the second resonant cavity is an integermultiple of the optical length of the first resonant cavity and the.Iaddend.net gain of the first resonant cavity containing said gainmedium is affected through interactions with the second cavity.